1. Field of the Invention
The present invention relates to a swash plate type compressor, especially to the improvement of a shoe to be used in the compressor.
2. Description of the Prior Art
Conventionally, a swash plate type compressor is used in such system as an air conditioning system of an automobile. The swash plate type compressor comprises a cylinder block which includes plural cylinder bores parallel to a rotary shaft; a swash plate which is rotated by the shaft within the cylinder block; pistons which are movably disposed in the cylinder bores, and shoes disposed between the pistons and the sliding surface of the swash plate which reciprocates the pistons. In the operation of the compressor, the rotary shaft rotates the swash plate rockingwise and further reciprocates the pistons in the cylinder bores, thus compressing the refrigerant in the bores.
In such compressor, a heavy load and a high sliding speed are applied to the sliding surface of the shoes which slide against the surface of the swash plate. Therefore, when a lubricating oil is not sufficiently supplied to the sliding surfaces of the shoes, as often observed just at the start of the compressor operation, seizure is likely to occur on the sliding surfaces, especially on the central parts. The main reasons why seizure is likely to occur on the central part of the sliding surface are described in (A), (B) and (C) that follow.
(A) Conventionally, a lubricating oil is included in the refrigerant of the swash plate type compressor to be supplied between the sliding surfaces of the swash plate and the shoes for seizure prevention. Increase of the lubricating oil will smooth the sliding motions of the swash plate and the shoes, but will considerably lower the refrigerating capacity of an air-conditioning system, since the amount of the refrigerant to be sent to a heat exchange is reduced by the increase of the lubricating oil and the heat absorbing capacity of the refrigerant is impaired by the oil addition. Therefore, the less the lubricating oil added to the refrigerant, the more the refrigerating capacity. For this reason, it is preferred to reduced the amount of the lubricating oil, which results in increased tendency to cause seizure.
(B) During operation of the compressor, friction heats the shoes up to temperatures as high as 200.degree. C. The heat generated in the shoes is dissipated from the peripheries of the shoes which are preferentially in contact with the refrigerant and the lubricating oil. Therefore, the heat is accumulated in the central parts of the shoes, which are likely to expand and protrude. Then, the protruded central parts are subjected to a higher load and are subject to seizure.
(C) During operation of the compressor, the film of the lubricating oil intervenes between the shoes and the swash plate and smoothes the sliding motions thereof. However, during stoppage of the compressor, lubricating oil forming the film flows down by gravity to produce an oil-less state in the interfaces between the swash plate and the shoes. Consequently, when the compressor is restarted after a long rest, the swash plate and the shoes slide against each other without any intervening oil film, and thus, seizure is likely to occur on the sliding surfaces of the shoes.